Waste heat boiler

ABSTRACT

A waste heat boiler has heat exchange tubes for indirect heat exchange of a relatively hot process gas and a cooling media, and a by-pass tube for by-passing a part of the process gas; a process gas collector collects and mixes a part of the heat exchanged process gas and at least a part of the by-passed process gas before the mix is lead via a control valve to the process gas outlet of the waste heat boiler together with the rest of the heat exchanged process gas.

BACKGROUND OF THE INVENTION

The present invention is directed to the recovery of waste heat from chemical reactions. More particularly, the invention relates to a waste heat boiler with improved control of cooling effect.

Waste heat boilers are most generally used for the generation of steam by waste heat recovered from hot process streams. Typically, those boilers are designed as shell-and-tube exchangers with a plurality of heat exchanging tubes arranged within a cylindrical shell.

Two basic types of shell-and-tube exchangers are employed in the industry, the water-tube type, in which water/steam mixtures flow through the tubes, and the fire-tube type having the heating process stream inside the tubes.

The characteristic components of the boiler are the tubes mounted in tubesheets at a front-end head and a rear-end head within the shell. In the fire-tube boilers steam production is accomplished on the shell side of the tubes by indirect heat exchange of a hot process stream flowing through the boiler tubes. The shell side is through a number of risers and down-comers connected to a steam drum, which may be arranged above or as an integral part of the boiler shell.

The mechanical design and, in particular, dimensioning of the heat exchanging surface in shell-and-tube exchanger type boilers represent certain problems. Fire-tube boiler applications involve high pressures on the shell side or on both sides, and considerable temperature differences between the shell side and the tube side. Particular considerations have to be given to fouling and corrosion characteristics of the process stream.

Boilers handling fouling and/or corrosive process streams must be designed to a higher duty than required when clean in order to allow for satisfying lifetime under serious fouling and/or corroding conditions. The heat transferring surface of the boiler tubes has further to be adapted to expected corrosion and fouling factors in the stream. To provide for a desired and substantially constant cooling effect during long term operation of the boilers, appropriate heat transfer and temperature control is required.

Conventionally designed boilers are equipped with a bypass of a relative large diameter tube (relative to the heat exchange tube diameter), which may be internal or external to the boiler shell. The by-pass is usually constructed as an insulated tube provided with a flow control valve. During initial operation of the boilers, part of the hot process stream is by-passed the heat transferring tubes to limit the heat transfer within the required level.

After a certain time, on stream fouling and/or corrosion of the tubes increase, leading to decreased heat transfer. The amount of by-passed process stream is then reduced, which allows for higher flow of the process stream through the heat transferring tubes to maintain the required cooling effect.

A major drawback of the known boilers of the above type is vigorous corrosion on the metallic surface of the by-pass, particularly the by-pass outlet and flow control valve, which are in contact with the un-cooled process stream at temperatures as high as 1000° C. or even higher. Known art has sought to solve this problem in various manners such as cooling the control valve with a cooling fluid or to avoid a hot by-pass stream and in stead divide the heat exchanger in different sections with different heat-exchange level and therefore different process gas outlet temperatures. Examples of known art are disclosed in U.S. Pat. No. 5,452,686A, US2007125317A, U.S. Pat. No. 4,993,367A, GB1303092A, U.S. Pat. No. 1,918,966A and EP0357907A.

SUMMARY OF THE INVENTION

An object of this invention is to avoid the drawbacks of the known waste heat boilers by providing a boiler of the shell-and-tube heat exchanger type with an improved heat transfer and temperature control.

A further object of this invention is to provide a waste heat boiler with a simpler and less expensive design than known art waste heat boilers.

A further object of this invention is to provide a waste heat boiler with a process gas by-pass tube and a control valve for simple control of the by-pass process gas stream and accordingly the process gas outlet temperature, without exposing the control valve to excessive temperatures leading to corrosion.

According to one embodiment of the present invention this is achieved by a waste heat boiler for heat exchanging a relatively hot process gas with a cooling media where the waste heat boiler comprises at least one shell part (a heat exchange section second shell part), and at least two tube sheets placed in an inlet end and an outlet end of the heat exchange section second shell part, whereby this second shell part and the two tube sheets enclose the heat exchange section of the waste heat boiler. A plurality of heat exchange tubes and at least one process gas by-bass tube are placed in the heat exchange section and are fixed in the first tube sheet near the first end of each tube and fixed in the second tube sheet near the second end of each tube. At least one cooling media inlet and at least one cooling media outlet are located on the waste heat boiler to enable a cooling media to flow into and out of the heat exchange section on the shell side of the tubes. The cooling media is thus enclosed by the second shell part and the first and the second tube sheet. A process gas inlet section is located near the first tube sheet, on the opposite side than the cooling media. The inlet section may further be enclosed by a first shell part at the process gas inlet end. A process gas outlet section is located near the second tube sheet also on the opposite side of the cooling media. Also the outlet section may further be enclosed by a third shell part. In the third shell part in the process gas outlet end, an outlet process gas collector is located such that it collects at least a part of the process gas exiting the at least one by-pass tube and also collects the cooled process gas exiting a part of the heat exchange tubes.

Process gas flows from the first shell part, process gas inlet end, to the heat exchange tube inlets and the by-pass tube inlet, through the heat exchange tubes and the at least one by-pass tube, out of the heat exchange tube outlets and the at least one by-pass process gas outlet to the third shell part, process gas outlet end. A cooling media flows into the heat exchange section via the cooling media inlet and is in contact with the shell side of the heat exchange tubes and can be in contact with the shell side of at least one by-pass tube before the cooling media exits the heat exchange section through the cooling media outlet. The process gas enters the process gas inlet section at a first temperature and exits the heat exchange tubes at a second relatively low temperature. The process gas exiting the by-pass tube has a third temperature which is lower or equal to the first temperature, but higher than the second temperature. The outlet process gas collector mixes at least a part of the by-pass process gas and a part of the heat exchanged process gas. Thus, the mixed process gas temperature has a fourth temperature which is higher than the second temperature, but lower than the third temperature. The amount of heat exchanged process gas and the amount of by-pass gas in this mix is composed such that this fourth temperature is low enough to prevent excessive corrosion of the control valve which is located downstream of the collected by-pass and heat-exchanged process gas. The control valve controls the amount of mixed process gas in the total process gas stream exiting the waste heat boiler. The total process gas stream exiting the waste heat boiler should be of a certain fifth temperature, which is higher than or equal to the second temperature, but lower than the fourth temperature. The control valve can control the volume flow of mixed process gas as compared to the total exiting process gas volume flow and thus control the fifth temperature even with varying second and fourth temperatures and volume flows. Accordingly, with a constant high first temperature and the aim of a constant low fifth temperature, but a rising second temperature due to fouling/corrosion of the heat-exchange pipes; the fifth temperature can be kept constant by use of the control valve to decrease the volume flow of the by-pass process gas having a fourth temperature.

According to a further embodiment of the invention, the mixing of the heat exchanged process gas and the by-passed process gas in the outlet process gas collector can be enhanced by mixing means located in the collector up-stream of the control valve. The mixing means can be of any known function and materials.

In a further embodiment of the invention, there is one by-pass tube and the collector collects the process gas exiting the one by-bass tube and the heat-exchanged process gas exiting at least one of the heat exchange tubes.

In another embodiment of the invention, the process gas inlet section is lined with a ceramic liner for protecting the first shell part from the relative hot process gas.

In a further embodiment of the invention also the by-pass tube, the mixing means and the process gas collector can be lined with a ceramic liner.

In an embodiment of the invention, the cooling media can be water or it can be steam. The cooling media can be water when entering the heat exchange section and a part of the water or all of the water can be heated by the indirect heat-exchange with the relative hot process gas such that all or a part of the cooling media exiting the heat exchange section via the cooling media outlet is steam.

In a further embodiment of the invention, the second shell part or both the first, the second and the third shell part can be substantially cylindrical. The cylindrical shape can be advantageous as it is a pressure robust and material saving shape. By substantial is meant any shape which is oblong in one cross sectional view and any shape which is not far from circular in another cross sectional view, such as circular, elliptic, square, pentagonal, hexagonal etc.

In a further embodiment of the invention, a plurality of heat exchange tubes are placed in a substantially circular array in the tube sheets and the by-pass tube or the at least one by-pass tube is placed substantially in the center of the array. By substantially is meant, that the location does not have to be mathematically accurate, the shapes can vary to a large extent as long as consideration to heat-exchange effectiveness and material costs are respected.

In an embodiment of the invention, the waste heat boiler is used in a process plant producing wet sulphuric acid.

FEATURES OF THE INVENTION

1. Waste heat boiler for heat exchanging a relatively hot process gas with a cooling media comprising

-   -   a shell part,     -   at least two tube sheets,     -   a plurality of heat exchange tubes,     -   at least one by-pass tube,     -   a heat exchange section enclosed by said shell part and said at         least two tube sheets     -   a process gas inlet section,     -   a process gas outlet section,     -   at least one cooling media inlet,     -   at least one cooling media outlet,         the relatively hot process gas enters the heat exchange tubes         and the at least one by-pass tube in the process gas inlet         section, flows through the heat exchange section where at least         the process gas flowing in the heat exchange tubes is in         indirect heat exchange with the cooling media and exits in the         process gas outlet section, wherein said waste heat boiler         further comprises a control valve and an outlet process gas         collector, said control valve is enabled to control the volume         stream of the process gas which flows through said outlet         process gas collector, the process gas collector collects at         least a part of the process gas exiting the at least one by-pass         tube and the cooled process gas exiting a part of the heat         exchange tubes.         2. Waste heat boiler according to feature 1, wherein the outlet         process gas collector further comprises mixing means located         up-stream of the control valve, for mixing the relatively hot         process gas exiting the at least one by-pass tube with the         cooled process gas exiting a part of the heat exchange tubes.         3. Waste heat boiler according to any of the preceding features         comprising one by-pass tube, wherein the outlet process gas         collector collects the process gas exiting the bypass tube gas         and the exiting process gas of at least one of the heat exchange         tubes.         4. Waste heat boiler according to any of the preceding features,         wherein the process gas inlet section is lined with a ceramic         liner.         5. Waste heat boiler according to feature 4, wherein further the         inside wall of the bypass tube is lined and at least part of the         outlet process gas collector is lined with a ceramic liner.         6. Waste heat boiler according to any of the preceding features,         wherein the cooling media is water or steam or both water and         steam.         7. Waste heat boiler according to any of the preceding features,         wherein said shell has a substantially cylindrical shape and         said at least two tube sheets have a substantially circular         shape.         8. Waste heat boiler according to any of the preceding features,         wherein said heat exchange tubes are arranged in a circular         array in the tube sheets and said by-pass tube is arranged         substantially in the centre of said array.         9. Process for heat exchanging a relatively hot process gas with         a cooling media in a waste heat boiler according to any of the         preceding features comprising the steps of,     -   providing the relative hot process gas to the process gas inlet         section,     -   providing the cooling media to the heat exchange section of the         waste heat boiler     -   heat-exchanging a first part of the relatively hot process gas         with the cooling media indirectly in the heat exchange tubes         located in said heat exchange section     -   by-passing a second part of the relatively hot process gas from         the process gas inlet section, through the heat exchange section         and to the process gas outlet section without substantial heat         exchange with the cooling media     -   collecting and mixing at least a part of the by-passed process         gas and a part of the cooled process gas in an outlet process         gas collector     -   controlling the volume stream of the collected and mixed process         gas with a control valve.         10. Use of a waste heat boiler according to any of the features         1-8 in a process plant producing Sulphuric Acid.

POSITION NUMBER OVERVIEW

-   100 Waste Heat Boiler, WHB -   110 First shell part, process gas inlet end -   111 Lining -   112 Process gas inlet section -   113 By-pass process gas inlet -   114 Heat exchange tube inlet -   115 First tube sheet, process gas inlet end -   120 Second shell part, heat exchange section -   121 Cooling media inlet -   122 Cooling media outlet -   123 Heat exchange tube -   124 Process gas by-pass tube -   125 Second tube sheet, process gas outlet end -   126 Heat exchange section -   130 Third shell part, process gas outlet end -   132 Process gas outlet section -   133 By-pass process gas outlet -   134 Heat exchange tube outlet -   135 Control valve, by-pass tube outlet -   136 Outlet process gas collector -   137 Outlet process gas mixing means -   138 mixed process gas outlet

FIG. 1 is a cross sectional view of the waste heat boiler 100 according to an embodiment of the invention. The waste heat boiler comprises a first shell part, process gas inlet end 110; a second shell part, heat exchange section 120 and a third shell part, process gas outlet end 130; all having a substantially cylindrical shape and substantially the same diameter, but as can be seen on the figure, not necessarily the same material thickness. The material thickness as well as the choice of material can be varied depending on the process conditions.

A first tube sheet, process gas inlet end 115 separates the first shell part from the second shell part. Likewise, a second tube sheet, process gas outlet end 125 separates the second shell part from the third shell part. Thus the first shell part and the first tube sheet encloses the process gas inlet section 112; the second shell part along with the first and the second tube sheet encloses the heat exchange section 126; and the third shell part and the second tube sheet encloses the process gas outlet section 132. The internal surface of the process gas inlet section can have a liner 111, for instance a ceramic liner to protect the internal surfaces from the high temperatures of the inlet process gas.

The first and the second tube sheets have corresponding bores to accommodate heat exchange tubes 123. The heat exchange tubes stretch at least from the first tube sheet through the heat exchange section and at least to the second tube sheet. The connection between each heat exchange tube and each of the tube sheets are made gas and liquid tight. Each heat exchange tube has a heat exchange tube inlet 114 located in the process gas inlet section and a heat exchange tube outlet 134 located in the process gas outlet section.

The first and the second tube sheets also have at least one corresponding bore for at least one process gas by-pass tube 124. In the embodiment of the invention according to FIG. 1 there is one process gas by-pass tube. The connection between the process gas by-pass tube and the first and the second tube sheet is made gas and liquid tight. The process gas by-pass tube has a by-pass process gas inlet 113 located in the process gas inlet section and a by-pass process gas outlet 133 located in the process gas outlet. The process gas by-pass tube can be provided with a lining (not shown) which can protect the tube from the relative high process gas temperatures and which may also reduce the indirect heat exchange between the cooling media and the by-passed process gas.

In the heat exchange section a cooling media inlet 121 provides fluid connection of a cooling media to the heat exchange section. The at least one cooling media inlet can be located in any position on the second shell part or even on the first or the second tube sheet, as long as fluid connection to the heat exchange section is provided. A location on the shell part of the heat exchange section is shown on FIG. 1. A cooling media outlet 122 located in fluid connection to the heat exchange section provides outlet of the cooling media from the heat exchange section.

Each of the heat exchange tubes and the process gas by-pass tube thus provides fluid connection from the process gas inlet section through the heat exchange section and to the process gas outlet section, thereby enabling the process gas to flow through the heat exchange section without direct contact to the cooling media. The process gas flowing in the heat exchange tubes is in indirect heat-exchange with the cooling media, whereas the part of the process gas which is by-passed, i.e. flowing in the process gas by-pass tube is in less or relative low or substantially no indirect heat-exchange with the cooling media: If the by-pass tube is not lined, the by-passed process gas will have some heat-exchange with the cooling media, but the heat-exchange in the by-pass tube will be relative lower than the heat-exchange in the heat exchange tubes due to the by-pass tube's higher volume to surface ratio. If the by-pass tube is lined, for instance with a ceramic liner, the indirect heat-exchange between the by-passed process gas flowing in the by-pass tube and the cooling media will be relative low or close to zero. In any case, the temperature of the heat-exchanged process gas exiting the heat exchange tube outlets is considerably lower than the temperature of the by-passed process gas exiting the by-pass process gas outlet.

In the process gas outlet section, an outlet process gas collector 136 is located. It collects the by-passed process gas and a part of the heat-exchanged process gas. In the embodiment according to FIG. 1, the process gas collector collects the by-passed process gas and the heat-exchanged process gas exiting the heat-exchange tubes located nearest to the by-pass tube in a circular array around the by-pass tube. Outlet process gas mixing means are located inside the process gas collector ensuring mixing of the by-passed and the part of the heat-exchanged process gas to an extent so the control valve, by-pass tube outlet 135 is not exposed to a critical amount of process gas with a temperature so high it will lead to substantial corrosion of the control valve.

Said control valve controls the amount of mixed process gas which is exited from the outlet process gas collector to the process gas outlet section. Thus, as the temperature of the heat-exchanged process gas is lower than the temperature of the mixture of the by-passed and the heat-exchanged process gas exiting the mixed process gas outlet 138, the control valve can vary the temperature of the process gas exiting the process gas outlet section within an interval between the temperature of the heat-exchanged gas and the mixed by-pass gas. Or more important, the control valve can keep the temperature of the process gas exiting the process gas outlet section on a certain level, even though the temperature of the heat-exchanged process gas vary due to for instance reduced indirect heat-exchange because of for instance fouling in the heat-exchange tubes.

FIG. 2 shows the waste heat boiler of FIG. 1 with the temperatures noted. The temperatures have the following relations:

t₁≧t₃>t₄>t₂

t₄>T₅≧t₂ 

1. Waste heat boiler for heat exchanging a relatively hot process gas with a cooling media comprising a shell part, at least two tube sheets, a plurality of heat exchange tubes, at least one by-pass tube, a heat exchange section enclosed by said shell part and said at least two tube sheets a process gas inlet section, a process gas outlet section, at least one cooling media inlet, at least one cooling media outlet, the relatively hot process gas enters the heat exchange tubes and the at least one by-pass tube in the process gas inlet section, flows through the heat exchange section where at least the process gas flowing in the heat exchange tubes is in indirect heat exchange with the cooling media and exits in the process gas outlet section, wherein said waste heat boiler further comprises a control valve and an outlet process gas collector, said control valve is enabled to control the volume stream of the process gas which flows through said outlet process gas collector, the process gas collector collects at least a part of the process gas exiting the at least one by-pass tube and the cooled process gas exiting a part of the heat exchange tubes.
 2. Waste heat boiler according to claim 1, wherein the outlet process gas collector further comprises mixing means located upstream of the control valve, for mixing the relatively hot process gas exiting the at least one by-pass tube with the cooled process gas exiting a part of the heat exchange tubes.
 3. Waste heat boiler according to claim 1, comprising one by-pass tube, wherein the outlet process gas collector collects the process gas exiting the bypass tube gas and the exiting process gas of at least one of the heat exchange tubes.
 4. Waste heat boiler according to claim 1, wherein the process gas inlet section is lined with a ceramic liner.
 5. Waste heat boiler according to claim 4, wherein further the inside wall of the bypass tube is lined and at least part of the outlet process gas collector is lined with a ceramic liner .
 6. Waste heat boiler according to claim 1, wherein the cooling media is water or steam or both water and steam.
 7. Waste heat boiler according to claim 1, wherein said shell has a substantially cylindrical shape and said at least two tube sheets have a substantially circular shape.
 8. Waste heat boiler according to any claim 1, wherein said heat exchange tubes are arranged in a circular array in the tube sheets and said by-pass tube is arranged substantially in the center of said array.
 9. Process for heat exchanging a relatively hot process gas with a cooling media in a waste heat boiler according to any of the preceding claims comprising the steps of, providing the relative hot process gas to the process gas inlet section, providing the cooling media to the heat exchange section of the waste heat boiler heat-exchanging a first part of the relatively hot process gas with the cooling media indirectly in the heat exchange tubes located in said heat exchange section by-passing a second part of the relatively hot process gas from the process gas inlet section, through the heat exchange section and to the process gas outlet section without substantial heat exchange with the cooling media collecting and mixing at least a part of the by-passed process gas and a part of the cooled process gas in an outlet process gas collector controlling the volume stream of the collected and mixed process gas with a control valve.
 10. Use of a waste heat boiler according to claim 1 in a process plant producing Sulphuric Acid. 